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BUG: KCSAN: data-race in prepare_signal / ptrace_attach

write to 0xffff8880380e7030 of 4 bytes by task 25662 on cpu 0:
 ptrace_attach+0x217/0x4b0 kernel/ptrace.c:407
 __do_sys_ptrace kernel/ptrace.c:1256 [inline]
 __se_sys_ptrace+0xc1/0x2e0 kernel/ptrace.c:1236
 __x64_sys_ptrace+0x51/0x60 kernel/ptrace.c:1236
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880380e7030 of 4 bytes by task 25663 on cpu 1:
 sig_ignored kernel/signal.c:116 [inline]
 prepare_signal+0x769/0xd80 kernel/signal.c:962
 __send_signal+0x64/0x780 kernel/signal.c:1082
 send_signal+0x39f/0x3d0 kernel/signal.c:1242
 do_send_sig_info+0x9b/0xe0 kernel/signal.c:1291
 do_send_specific kernel/signal.c:3815 [inline]
 do_tkill kernel/signal.c:3841 [inline]
 __do_sys_tkill kernel/signal.c:3876 [inline]
 __se_sys_tkill+0x12b/0x180 kernel/signal.c:3870
 __x64_sys_tkill+0x2d/0x40 kernel/signal.c:3870
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 25663 Comm: syz-executor.4 Not tainted 5.10.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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